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Mechanical engineering |Машиностроение

Lesson 6

Read the text: Broaching machines

Broaching is one of the most productive precision-machining processes known. It's a high-production, metal-removal process that sometimes is required to make one-of-a-kind parts. It's at its best when machining simple surfaces or complex contours. Its recent successes include such dissimilar items as high-precision computer parts and massive locomotive bull gears. Broaching is similar to planning, competes with milling and boring, and gives turning and grinding stiff competition. Properly used, broaching can greatly increase productivity, hold tight tolerances, produce precision finishes, and minimize the need for highly skilled machine operators.

Tooling is the heart of any broaching process. The broaching tool is based on a concept unique to the process - rough, semi-finish, and finish cutting teeth combined in one tool or string of tools. Broach tools frequently can finish-machine a rough surface in a single stroke. In its simplest form, a broach tool resembles a wood rasp. It is a slightly tapering round or flat bar with rows of cutting teeth located along the tool axis. In advanced forms, extremely complex cross-sections and tooth designs may be found. However, the basic axial, multi-toothed tool shape remains.

For exterior or surface broaching, the broach tool may be pulled or pushed across a work piece surface; or the surface may move across the tool. Internal broaching requires a starting hole or opening in the work piece so the broaching tool can be inserted. The tool, or the work piece, is then pushed or pulled to force the tool through the starter hole. The final shape may be a smoother, flatter surface, larger hole, complex splinted, toothed notched curved, helical, or some other irregularly shaped section. Almost any irregular cross-section can be broached as long as all surfaces of the section remain parallel to the direction of broach travel. The exceptions to this rule are uniform rotating sections such as helical gear teeth, which are produced by rotating the broach tool as it passes the work piece surface. Blind holes or holes with limited depth can also be broached with punch broaches which are pushed with limited travel.

The length of a broach tool or string of tools is determined by the amount of stock to be removed, and limited by the machine stroke, bending moments (in a push broach), stiffness, accuracy, and other factors. A pull broach is usually limited to 75 times the diameter of the finishing teeth. Broaching tools can be as small as 0.050 in. or as large as 15 to 20 in. in diameter.

Broach teeth usually are divided into three separate sections along the length of the tool: the roughing teeth, semi-finishing teeth, and finishing teeth. The first roughing tooth is proportionately the smallest tooth on the tool. The subsequent teeth progressively increase in size up to and including the first finishing tooth. The difference in height between each tooth, or tooth rise, usually is greater along the roughing section and less along the semi-finishing section. All finishing teeth are the same size.

The land supports the cutting edge against stresses. A slight clearance or back off angle is ground onto the lands to reduce friction. On roughing and semi-finishing teeth, the entire land is relieved with a back off angle. On finishing teeth, part of the land immediately behind the cutting edge is often left straight so that repeated sharpening (by grinding the face of the tooth) will not alter the tooth size.

1. Match the left part with the right:

2. Complete the sentences with the suggested words: semi-finishing section;

 ength; size; finishing tooth; tool; roughing; broach

_____teeth usually are divided into three separate sections along the____ of the tool: the roughing teeth, semi-finishing teeth, and finishing teeth. The first____tooth is proportionately the smallest tooth on the___. The subsequent teeth progressively increase in size up to and including the first____. The difference in height between each tooth, or tooth rise, usually is greater along the roughing section and less along the____. All finishing teeth are the same ____.